1. Field of the Invention
The present invention relates to a method of reducing NOx emissions from coal fired furnaces. More particularly it relates to the retrofitting of existing pulverized coal burners for low NOx emissions.
2. Description of the Prior Art
Nitric oxide (NO) is an air pollutant. In many areas of the United States, as well as other countries, methods are sought to reduce its concentration level in flue gases emitted from coal-fired boilers. In the combustion of fuel in the boilers, one problem is the production of nitric oxide due to oxidation of both fuel-bound nitrogen and nitrogen entering with the combustion air.
A portion of the nitric oxide produced by a burner oxidizes to form nitrogen dioxide (NO.sub.2) downstream of the combustion process, as well as in the atmosphere. Consequently, production of nitric oxide at the burner results in both NO and NO.sub.2, commonly called NOx, being emitted into the atmosphere.
In pulverized coal combustion as practiced in boilers, kilns, and other combustion devices, the pulverized coal is generally conveyed to the burners by the "primary" air stream. The primary air in many cases is preheated, dries the coal and carries the coal out of the pulverizer. The ratio of primary air to coal is typically between 1 and 3 on a weight basis to best accomplish these functions.
As the coal and primary air stream enters the furnace via the burner, heat from downstream combustion is transported by recirculated gases and radiation back to the incoming coal particles causing them to heat and devolatilize. The volatiles that are released from the coal particles mix with the primary air, and the temperature and fuel/air ratio of the mixture eventually become sufficient for ignition to occur. The ignition stability of the burner thus depends mainly upon this process of heat transfer from the primary flame zone, devolatilization of the coal, and ignition of the coal volatiles-primary air mixture and/or the solid coal.
As mentioned above, the region immediately following the ignition zone of the burner, in which the coal devolatilization is completed and the volatiles are burned, is generally termed the "primary flame" zone. In this zone, the bulk of the combustion air, i.e., the "secondary" air which is admitted separately from the primary air, mixes with the fuel and burns. The primary flame zone is followed by a char burnout zone in which the devolatilized coal particles are burned in an atmosphere of typically 15% to 25% excess air (i.e., 3% to 5% O.sub.2).
Formation of NO occurs in both the primary flame zone and the char burnout zone. In the primary flame, NO forms primarily from oxidation of volatilized organic nitrogen compounds. In the char burnout zone, NO forms primarily by oxidation of organic nitrogen compounds in the char, and to a minor extent by oxidation of nitrogen in the air. These three NO formation mechanisms can be summarized as follows:
______________________________________ Primary Flame: Volatile N + O.sub.2 .fwdarw. NO + O (1) Char Burnout: Char N + O.sub.2 .fwdarw. NO + O (2) Air Nitrogen N.sub.2 + O .fwdarw. NO + N (3) Followed by N + O.sub.2 .fwdarw. NO + O (4) ______________________________________
Work by Pohl and Sarofim, reported in "Devolatilization and Oxidation of Coal Nitrogen," showed that a major fraction (approximately 70%) of NO formed in pulverized coal combustion is due to oxidation of volatile fuel nitrogen, i.e., equation 1 above.
One method that is commonly applied to reduce NO formation in pulverized coal firing is the use of low-NOx burners. Low-NOx burners reduce NO formation by delaying the mixing of secondary air into the primary flame. Delay of secondary air mixing produces a lower air/fuel ratio (i.e., air/volatiles ratio) in the primary flame, thus reducing the amount of NO formed from volatile fuel nitrogen. The fact that lowering the air/fuel ratio in the primary flame reduces NO formation is demonstrated by the work of Kawamura and Frey, "Current Developments in Low-NOx Firing Systems," in which the primary air was lowered, causing a reduction in NO formation.
From a practical standpoint, retrofitting low NOx burners to existing pulverized coal fired boilers is the most common approach for implementing the above strategy and for meeting future "acid rain" limitations for NOx emissions. However, while low NOx burners may reduce NOx emissions by 50to 60% compared to conventional pulverized coal burners, retrofitted low NOx burners can cause a number of serious operational problems. These problems include flame impingement, slagging, and incomplete carbon burnout. To overcome these problems, operators will use less coal, a practice known as derating of the boiler. The Electric Power Research Institute (EPRI) has estimated that only 30% of the coal fired utility boilers in the U.S. can be retrofitted with low NOx burners without significant and costly derating penalties. In addition to derating, low NOx burners can cause other operation difficulties. These include poor load following, poor flame holding and stability, and increased sensitivity to upsets.
Additionally, the fuel fed to low NOx burners cannot be adjusted from the different mills, which causes problems in meeting load where mills are out of service. Thus there is a need for a system that will allow boilers retrofitted with low NOx burners to operate as originally designed with conventional burners while achieving NOx emission limitations.
Low NOx burners typically achieve their desired characteristic by burning a portion of the coal in a fuel rich environment followed by mixing more air into the flame which completes the burn out. The portion of the coal burned in the fuel rich environment produces only a very small amount of NOx. In the fuel rich portion there are fewer oxygen molecules and oxygen atoms to react with nitrogen atoms which are present primarily from the coal or with nitrogen molecules which are part of the air.
Unfortunately, the requirement for delayed mixing of part of the air makes the flame longer. In some furnaces the distance from the firing wall to the opposite wall is not adequate to accommodate the long flame. The flame may impinge on the opposite wall causing slagging and corrosion problems and the wall may quench the flame increasing carbon carryover.
Alternately, the flame may be deflected upward and carry much of the combustion energy into the upper part of the furnace resulting in excessive superheater or reheater temperatures. The accommodation to these problems is usually to derate the units, that is, operate them at part load. Thus there is a need for a system which will overcome the problems associated with a longer flame without derating the units.